1999 Ford Mustang Shakes at speeds from 70-85 mph

I have a 1999 Mustang GT that has undergone extensive modifications. A year ago longtube headers, aluminum driveshaft, 3.73 gears, 18 inch FR500 wheels, and complete rear axel maintainence (shafts and bearings). About 3 months ago, I had a new DSS Racing engine that was balanced and blueprinted with blower, five speed transmission, chromoly front K-member, and Tokico coilover installed and dyno'd. Because of the K member and coilover, I also had the front end aligned. My problem is this, between 70-80 mph, I have a very annoying vibration coming from what appears the passanger front wheel area. I also feel it through the seat of my pants, steering wheel, and foot area. If I feel the carpet area around where the tranny sits, it comes from there as well. Above 80 mph, the vibration or what feels like almost a wheel hop, turns into a fast vibrating hum or almost what sounds like a fast grind. Not like metal to metal, hell its hard to explain. I though it might be worn tires, so I bought a new set of those. Had the wheels balanced to extreme tolerances and then had the tires put on and had them road forces to precise tolerances. Problem still there, took it back and had them do it again, problem still there, had the alignment redone. It was perfect. Had driveshaft rotated and yoke inspected, problem still there. Any thoughts on where to start next would be most helpful.

Thank you